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    <title>Document</title>
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<script>
    var num=1;
    var str="hello";
    var str1="world";
    var b1=true;
    /*变量声明了，但是没有赋值的时候*/
    var u1=undefined;
    //null表示一个空对象
    var n1=null;
    //javascript中变量的数据类型取决于它存储的值
    num="world";
    console.log(typeof num);
</script>
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